Assessing the dog as a model for human total hip replacement. Analysis of 38 canine cemented femoral components retrieved at post-mortem.

Post-mortem retrieval of canine, cemented femoral components was analysed to assess the performance of these implants in the dog as a model for human total hip replacement (THR). Mechanical testing and radiological analysis were performed to determine the stability of the implant and the quality of the cement. Thirty-eight implants from 29 dogs were retrieved after time intervals ranging from 0.67 to 11.67 years. The incidence of aseptic loosening was 63.2%, much higher than in human patients (6% in post-mortem studies). Failure of the femoral implants began with debonding at the cement-metal interface, similar to that in implants in man. The incidence of aseptic loosening was much lower in bilateral than in unilateral implants. Significant differences were observed for three different designs of implant. While the dog remains the animal model of choice for THR, results from this study provide insight into interspecies differences in the performance of implants. For example, the performance of THR in dogs should be compared with that in young rather than in elderly human patients.

Effect of early axial dynamization on tibial bone healing: a study in dogs.

Early axial dynamization and its effect on experimental tibial bone healing was compared with healing under rigid fixation in a time-sequenced manner using dogs. An external fixator that could be rigidly locked or set to allow free axial movement while preventing bending and shear was used. Both tibias were osteotomized and externally fixed, leaving a gap between bone ends of 2 mm. At 1 week, one side was dynamized, whereas the other side was kept rigidly locked as a control. Dogs were euthanized at 1 day and 1, 3, 5, 8, and 11 weeks after dynamization. The outcome measures were static and dynamic load-bearing, periosteal callus development, new bone formation, callus tissue composition, and mechanical strength. Load bearing was higher on the dynamized limbs during standing for the first 5 weeks and during gait for the first 3 weeks after dynamization compared with the controls. Maximum periosteal callus size was reached faster and was distributed more symmetrically on the dynamized side. The periosteal callus area decreased at 12 weeks on the dynamized sides, but there was no significant change in the area on the control sides. Endosteal new bone formation and bone density decreased between 9 and 12 weeks only on the dynamized sides. The dynamized side showed a significantly higher torsional stiffness at 6 weeks than did the controls. There were no significant differences between dynamized and control tibias at other times. Maximum torque also tended to be higher on the dynamized sides at the same time. Early axial dynamization appeared to accelerate callus formation and remodeling and to provide higher mechanical stiffness during early stages of bone healing.

Owner assessment of the outcome of total hip arthroplasty in dogs.

OBJECTIVE: To evaluate owners' perceptions of the outcomes of dogs that have undergone total hip arthroplasty (THA). DESIGN: Survey. SAMPLE POPULATION: Owners of 145 dogs that had undergone THA. PROCEDURE: Surveys were mailed to owners of 353 dogs that underwent THA between 1982 and 1996. Owners were asked to answer multiple-choice questions and provide written comments. RESULTS: The response rate was 41%. For owners who responded to the survey, time from surgery to completion of the questionnaire ranged from 6 months to 11 years. Overall, 122 respondents (84.1%) rated results of THA in their dog as excellent or good. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that owners are generally satisfied with the results of THA in their dogs.

The effect of knotting method on the structural properties of large diameter nonabsorbable monofilament sutures.

OBJECTIVE: To evaluate the effect of knotting method on the mechanical properties of large diameter nonabsorbable monofilament suture materials. STUDY DESIGN: In vitro mechanical evaluation. METHODS: A conventional square knot was compared with the surgeon's knot, sliding half-hitch, and clamped square knot. Knotted suture loops were created in a uniform manner and acutely tensioned to failure (20 mm/min loading rate; n = 20 per knot type for each material). Stiffness, yield, and failure characteristics of USP #2 nylon, #2 polybutester, #2 polypropylene, 27 kg test monofilament nylon fishing line, and 27 kg nylon leader material were evaluated. RESULTS: Compared with a conventional square knot, a surgeon's knot decreased stiffness for #2 polypropylene, 27 kg fishing line, and 27 kg leader (P < .05). A sliding half-hitch weakened all materials except 27 kg leader (P < .05). Clamping the first throw of a square knot increased the stiffness of 27 kg leader loops (P < .05). CONCLUSIONS: Based on clinically relevant parameters (stiffness and yield), knotting method had no effect on #2 nylon and #2 polybutester. The surgeon's knot is not recommended for #2 polypropylene and 27 kg fishing line and leader material. A sliding half-hitch decreased the yield of leader material. Clamping the first throw of a square knot had no adverse effects on acute properties of tested materials; it increased the stiffness for leader material. CLINICAL RELEVANCE: Knotting method does influence the structural properties of suture materials and should be considered when tying knots under tension.

Biologic attachment of an allograft bone and tendon transplant to a titanium prosthesis.

An Achilles tendon allograft with its bony insertion was used to bridge a Titanium implant, containing an endoprosthetic tendon anchor, and the sheep biceps muscle. Twelve sheep were operated on unilaterally and followed up clinically and histologically for 2, 4 (n = 2), 8, and 12 months (n = 4). Full function of the front limb was regained after 8 to 12 weeks. There were no signs of mechanical loosening at all times. The morphologic changes at the bone block and implant fixation site were an initial revascularization of the allograft bone, which was observed at 2 months and enhanced at 4 months but occurred without any evidence of bone remodeling. This was changed in all specimens taken at 8 and 12 months where intensive new bone development, remodeling, and bone ingrowth in the titanium implant was found. Bone mass was shifted significantly to the tendon insertion half of the bone block because of a creeping substitution of the cancellous allograft bone and bone ingrowth to the implant. Overall bone mass slightly decreased with time but resorption of allograft bone outweighed new bone development only at lesser loaded areas. Transplantation of a bone and tendon allograft to an implant resulted in a revitalized, mechanically stable, and biologically anchored compound.

Aseptic loosening of the femoral implant after cemented total hip arthroplasty in dogs: 11 cases in 10 dogs (1991-1995).

Aseptic loosening of the femoral implant (ALFI) was diagnosed in 10 dogs that had undergone cemented total hip arthroplasty (THA). One dog had bilateral ALFI. Loosening developed at the stem-cement interface a mean of 30 months after THA. The most common clinical sign was intermittent subtle or non-weight-bearing lameness. On radiographs obtained after THA, the distal stem tip was in contact with the cortical endosteum in all dogs. Radiographic changes at the time of diagnosis of ALFI included asymmetric periosteal reaction along the femoral diaphysis (n = 11), radiolucent zone at the stem-cement interface (6), altered implant position (4), and femur fracture (1). Surgical revision resulted in a good or excellent outcome in 9 dogs. In 1 dog, the implant became infected. In another, aseptic loosening recurred. Aseptic loosening of the femoral implant was significantly more common when initial positioning of the implant resulted in contact between the distal tip of the implant and cortical endosteum than when there was no contact.

Tendons attached to prostheses by tendon-bone block fixation: an experimental study in dogs.

To develop a method of tendon attachment to a metallic endoprosthesis, we evaluated fixation strength, clinical function of the tendon, and morphological changes in an experimental model. The canine supraspinatus tendon was removed from the greater tubercle of the humerus and attached to a titanium prosthesis. In 12 animals, the bone block underlying the tendon insertion was preserved and attached in one limb; the soft part of the tendon was attached directly to the prosthesis in the contralateral limb. Fixation strength was evaluated after 16 weeks of in vivo implantation (12 specimens) and compared with the in vitro fixation strength (12 specimens) and with intact normal controls (six specimens from cadavera). Function of the tendon in vivo was evaluated by force-plate analysis (at 3-week intervals). All specimens were evaluated histologically. Sixteen weeks after surgery, the tendon-bone block attachment was significantly stronger (mean, 16%) than the direct tendon attachment and not significantly different from the normal control, and the direct tendon attachment was significantly weaker (mean, 68%) than the normal control. There was significantly more weight-bearing on the limbs with a tendon-bone block attachment than on the limbs with a direct tendon attachment at both 3 and 6 weeks postoperatively. Both front legs showed increased weight-bearing with time, but the differences were not statistically significant. Anchorage by tissue ingrowth to the titanium prosthesis was found consistently--there was bone ingrowth in the tendon-bone block attachments and fibrous tissue ingrowth in the direct tendon attachments. When a bone block was preserved, the strength and stiffness were comparable with those of a normal tendon insertion.(ABSTRACT TRUNCATED AT 250 WORDS)

Histomorphometry of hydroxyapatite coated and uncoated porous titanium bone implants.

Two different groups of hydroxyapatite (HA) coated and uncoated porous titanium implants, 250-350 microns and 500-700 microns diameter beads, were press-fitted into femoral canine cancellous bone. After 12 wks, the dogs were killed and histomorphometric backscattered electron microscopy studies were carried out. Comparing HA coated versus uncoated implants in the 250-350 microns bead diameter group, the percentage of bone (P = 0.01) and bone index (P = 0.01) were higher in the HA coated implants. Comparing HA coated versus uncoated implants in the 500-700 microns bead diameter group, bone ingrowth (P = 0.01) and bone depth penetration (P = 0.008) were higher in HA coated samples. It can be concluded that the HA coating was an effective method for improving bone formation and ingrowth in the porous implants.

Fixation of canine tendons to metal.

For the purpose of developing a method to attach tendons directly to the prosthesis, canine supraspinatus tendons were attached in vitro to a metallic surface, using 3 different fixation devices: a spiked polyacetal washer (Synthes), a spiked soft tissue fixation plate (Synthes), and a newly designed Enhanced Tendon Anchor (ETA), which straddled the tendon with interlocking spikes oriented at a 20-degree angle. 2 methods were used: 1) the tendon was fixed directly to the metallic surface, or 2) a bone block containing the tendon insertion was fixed to the metallic surface. The specimens were tested for initial fixation strength in tension to failure; intact bone-muscle-tendon-bone units were used as controls. Bone block fixations were stronger than direct tendon fixations when the spiked washer or the ETA was used; this was not true of the fixation plate. The ETA was stronger than the other techniques in ultimate strength in both direct tendon fixation and bone block fixation. The soft tissue fixation plate was found to be weaker than the other techniques in bone block fixation.

Ex vivo biomechanics of Kirschner-Ehmer external skeletal fixation applied to canine tibiae.

The purpose of this study was to determine the respective contribution of each of the following parameters to the compressive, bending, and torsional rigidity of the Kirschner-Ehmer (KE) external fixation splint as applied to canine tibiae with an osteotomy gap: bilateral versus unilateral splints; increasing the number of fixation pins; altering the diameter of fixation pins and side bars; decreasing side bar distances from the bone; increasing pin separation distances in each pin group; decreasing distances between pin groups; altering pin clamp orientation; and altering side bar conformation. Bilateral splints were 100% (mean) stiffer than unilateral splints, with stiffness enhanced to the greatest extent in mediolateral bending and torsion. Increasing pin numbers stiffened both bilateral (mean, 41%; 8 versus 4) and unilateral splints (mean, 14%; 8 versus 4). Medium KE splints were 85% (mean) stiffer than small KE splints. Decreasing side bar distances to the bone from 1.5 cm to 1.0 cm to 0.5 cm increased stiffness of both bilateral and unilateral splints by a mean of 13% to 35%. Widening pin spacing from 1.67 cm to 2.5 cm increased stiffness in craniocaudal bending only (56% increase, bilateral splints; 73% increase, unilateral splints). Decreasing the distance between pin groups from 5.84 cm to 2.5 cm increased stiffness in torsion between 23% (unilateral splints) and 45% (bilateral splints) and decreased stiffness of unilateral splints by 29% in craniocaudal bending. Altering pin clamp configuration so that the bolts of the clamp were inside the side bar rather than outside the side bar increased stiffness in axial compression only (73% increase, bilateral splints; 54% increase, unilateral splints). Conforming the lateral side bar to the tibiae increased only axial compressive stiffness by 77% but was no different than placing the clamps inside the side bars of an unconformed bilateral splint. These results quantify the relative importance of specific parameters affecting KE splint rigidity as applied to unstable fractures in the dog.

Effects of axial dynamization on bone healing.

The effects of early dynamization (physiologic axial compression) on canine fracture healing at six weeks were studied. Bilateral transverse mid-tibial osteotomies were created and initially stabilized with a 2-mm gap using relatively rigid external fixators. Seven days after osteotomy, the telescoping mechanism of one of the fixators on each dog was released (dynamized), resulting in physiologic loading of the osteotomy, while the contralateral fixator remained locked as a rigid control. The dynamized osteotomy closed, and increased functional weight bearing resulted from 3 weeks on. Radiographically the amount of periosteal callus increased over time, but no difference in callus size was seen between the dynamized fractures and the controls. Torsional mechanical testing found the dynamized osteotomies to be significantly stiffer, and they tended to tolerate more maximum torque than the controls. Microscopic evaluation found no difference in the volume of the periosteal and endosteal calluses or in the tissues constituting them. However, a significantly greater proportion of the dynamized osteotomy gap was filled with new bone. These results suggest that dynamization in this delayed union model improved fracture healing by reducing fracture gap size and increasing weight bearing, not by altering the pathway of fracture healing.

Instrumentation for external fixation.

External skeletal fixation is a very useful technique for managing many orthopedic problems in veterinary practice. The Kirschner apparatus has been the most widely used fixator for many years in veterinary orthopedics because of its versatility, simplicity, and economy in use. The medium-sized device has the widest indications and is easiest to begin with. The small size can be acquired later for use on cats and small dogs. The new "raised thread" fixation pin designs improve bone-pin integrity and can be used in combination with nonthreaded pins to decrease the incidence of postoperative complications and for economy. Acrylic-pin external fixators are particularly useful for treatment of mandibular fractures and transarticular application since they allow nonlinear placement of fixation pins in highly contoured bones. A commercial system, currently being developed, will have all the equipment and materials necessary for their application in a convenient kit. Circular fixators (Ilizarov design) use thin K wires placed under tension to replace rigid fixation pins. Their unique adjustability characteristics make them useful in the treatment of limb deformity and shortening. Many other human fixators can be used for veterinary application if the basic principles of fixators are followed. Instrumentation required for external fixator application include a pin driver, pin cutter, and wrenches. Although surgical versions of these materials are available, less expensive alternatives are available using gas sterilization instead of autoclaving. The use of selected orthopedic instrumentation such as a periosteal elevator, bone clamps, and curets will facilitate fracture management.

Complications of external fixation. A problem-oriented approach.

External fixation is an effective method of stabilizing many fractures in veterinary orthopedics, however, it is not foolproof. Although careful attention to the principles of fixator selection and application is the optimal way to minimize complications, they do still occur. Prompt recognition of these complications and appropriate treatment, however, will minimize their effects and result in the successful outcome of fracture healing and normal limb function.

Modified "cup-patch" ileocystoplasty for urinary bladder reconstruction in a dog.

Full-thickness wall necrosis involving 90 to 95% of the urinary bladder was diagnosed in a 6-month-old Golden Retriever 2 days after ovariohysterectomy. An isolated, vascularized segment of ileum denuded of mucosa was used to reconstruct the urinary bladder. Serial excretory urography over 1 year indicated gradual enlargement of the bladder, with development of a smooth mucosal surface. At 6 months after reconstructive surgery, the dog was voiding urine 2 to 3 times/d and was continent. Results of renal function testing, urinalysis, and bacteriologic culture of urine were all considered normal on follow-up examinations. The cause of bladder wall necrosis was never determined. Ileocystoplasty as described herein appears to be an effective urinary bladder reconstructive procedure in dogs whenever the bladder neck, proximal portion of the urethra, and their neurovascular supply can be spared.

Comparison of the strength and holding power of 4 pin designs for use with half pin (type I) external skeletal fixation.

The strength and holding power of four pin designs for use with half pin (type I) external skeletal fixation were evaluated. Pins that were tested were fully threaded, nonthreaded, two cortices partially threaded, and one cortex partially threaded. The study involved three parts: (1) resistance of the pins to axial extraction immediately after insertion; (2) resistance of the pins to axial extraction 8 weeks after being inserted into the tibiae of live dogs; and (3) resistance of the pins to bending load. Pins with threads engaging two cortices were more resistant to axial extraction than nonthreaded pins in both the acute (p less than 0.0001) and chronic (p less than 0.0001) studies. Nonthreaded pins were more resistant to bending than fully threaded and two cortices partially threaded pins (p less than 0.0005). One cortex partially threaded pins possessed similar bending strength to nonthreaded pins (p = 0.21) and had 5.3 times more resistance to axial extraction in the acute study (p less than 0.0001) and 6.9 times more in the chronic study (p less than 0.0001). Though one cortex partially threaded pins were not as resistant to axial extraction as pins with threads engaging two cortices (p less than 0.0001), they were more resistant to bending loads (p less than 0.0005). Loss of holding power and pin failure are two of the most serious problems associated with fracture stabilization using external skeletal fixation. The results of this study suggest that one cortex partially threaded pins are better at maintaining holding power and resisting bending and breaking than nonthreaded pins.(ABSTRACT TRUNCATED AT 250 WORDS)

Type I biplanar configuration of external skeletal fixation: application technique in nine dogs and one cat.

Mechanical attributes and techniques of applying the type I biplanar (quadrilateral) configuration of external skeletal fixation are reported. The apparatus was used clinically on 9 dogs and 1 cat with long bone fractures. Bony union occurred in these animals, but was slightly delayed in one. Return to clinical function of the limb generally was good to excellent. The instances of delayed union and poor clinical results are discussed to emphasize the use of good techniques of application and case selection.